This invention relates generally to coherent light sources and particularly to laser diodes. Still more particularly, this invention relates to apparatus and methods for controlling the emission wavelength and output intensity of laser diodes.
The development and practical implementation of sensing systems that require an optical signal input and high data rate fiber optic communication systems require stability in the optical pulses input to the optical fibers. Such systems may use semiconductor diode lasers as light sources.
There are at least three groups of laser diodes that are classified according to structure. Simple diode lasers are called homostructure lasers because they are made of a single semiconductor material. A homostructure laser diode may comprise, for example, regions of n-type and p-type gallium arsenide. The combination of electrons injected from the n-region into the p-region with holes, or positive charge carriers, in the p-region causes the emission of laser light. All laser diodes include two polished parallel faces that are perpendicular to the plane of the junction of the p-type and n-type regions. The emitted light reflects back and forth across the region between the polished surfaces and is consequently amplified on each pass through the junction.
A typical single heterostructure semiconductor laser includes an additional layer of aluminum gallium arsenide, in which some of the gallium atoms in the gallium arsenide has been replaced by aluminum atoms. Injected electrons are stopped at the aluminum gallium arsenide layer, which causes the emission of a higher intensity laser light than ordinarily occurs with a homostructure diode laser.
A typical double heterostructure semiconductor laser includes three layers of gallium arsenide separated by two layers of aluminum gallium arsenide. Preselection of either n-type or p-type materials cause further increases of the intensity of the emitted laser beam.
The intensity and wavelength of the light emitted from a laser diode varies as functions of the operating temperature and the injection current applied thereto in order to supply electrons thereto. Effective use of a laser diode as a light source often requires an output of known intensity and wavelength. Both the intensity and the wavelength are non-linear functions of the injection current and the operating temperature of the laser diode.
Previous methods of regulating the emission wavelength or intensity have used univariant control systems where either the temperature or the injection current is varied to adjust the wavelength. Such systems can exhibit damped harmonic oscillator coupling between the injection current and temperature. Prior control systems that regulate the intensity and the wavelength have the disadvantage of requiring excessively long times to reach the desired wavelength and intensity. In some severe operational situations, the desired values of wavelength and intensity are never obtained because the system oscillates about the desired values.